Power semiconductor switching devices are widely in use for a large range of power applications. The power semiconductor switching devices with which we are concerned typically have a current-carrying capability of greater than 1 amp and are operable with a voltage of greater than 100 volts, for example devices that are able to carry currents of greater than 10 amps, 50 amps or 100 amps and/or are able to sustain a voltage difference across the device of greater than 500 volts or 1 KV.
Examples of such devices include insulated-gate bipolar transistors (IGBTs), as well as FETs such as MOSFETS (vertical or lateral) and JFETs, and potentially devices such as LILETs (lateral-inversion layer-emitter transistors), SCRs and the like. The techniques we will describe are not limited to any particular type of device architecture and thus the power-switching devices may be, for example, either vertical or lateral devices; they may be fabricated in a range of technologies including, but not limited to, silicon and silicon carbide.
Switching devices of this type have applications which include switching in high-voltage transmission lines, in particular dc transmission lines of the type which may, for example, carry power from an offshore wind installation, and medium voltage (for example greater than 1 KV) switching for motors and the like, for example locomotive motors. In particular, IGBTs are used to control large currents by the application of low-level voltages or currents, some IGBTs having ratings of, e.g., 1600V and 1200 A.
In applications of this type typically tens or hundreds of devices may be connected in series and/or parallel to operate at the desired voltages/currents. Controlling the switching of such devices presents particular problems, because the electrical environment is relatively noisy, operating conditions such as load currents and temperatures are continuously changing, and because the voltages/currents being switched are large, leading to a significant risk of device failure. Moreover when one device in such a system fails, other switching devices in the system can easily fail as a consequence.
Generally, systems for switching medium or high voltages are more easily constructed using multiple devices arranged in a series topology, in order to avoid use of higher power devices that are more costly and/or have slow switching speed. In an example inverter, IGBTs may be stacked and placed between power supply rails to form a phase leg as shown for example in FIG. 1a, which may represent a single leg inverter or a phase leg of a multiple leg inverter. FIG. 1b shows a multiple phase leg inverter having two IGBTs stacked in each of the upper and lower sides of each phase leg. The provision of a plurality of IGBTs in series allows the overall voltage, e.g., across an inverter phase leg or across IGBTs of a lower or upper side of an inverter phase leg, to be split across the IGBTs, allowing lower voltage IGBTs to be used and/or application of a higher overall voltage. Nevertheless, difficulties remain even when a series connection of IGBTs is used, for example in relation to reliability as indicated above.